The invention relates to a method for operating an internal combustion engine, particularly of a motor vehicle, operated with a gas, in particular natural gas, as fuel, a lambda value of an air/fuel mixture being regulated by means of lambda control as a function of a signal from a lambda probe.
The subject of the present invention is also a computer program and a computer program product, which are suitable for carrying out the method.
With a view to the international effort to reduce CO2 emissions and adhere to ever more stringent exhaust gas limit values, natural gas is acquiring increasing importance as an alternative fuel for vehicles. In this case, natural gas can be stored both as liquid at low temperature or compressed under high pressures. In vehicles, natural gas is used in practice in almost all applications in compressed form, that is to say in gas form. In most systems, gas injection takes place into the suction pipe, as in conventional gasoline injection. The injection valves are in this case supplied with natural gas via a low-pressure common rail and inject the natural gas into the suction pipe intermittently. By the fuel being introduced completely in gas form, mixture formation is improved, since natural gas does not condense on the suction pipes and also no wall film is built up. Particularly during warming up, this has a beneficial effect on emissions. The extremely high knock resistance of natural gas allows a higher compression of the engine to approximately 13:1 in order to improve engine efficiency. A natural gas engine is therefore also ideally suitable for supercharging. The CO2 emissions which are approximately 25% lower, as compared with conventional gasoline engines, are especially advantageous in natural gas vehicles. These emissions result from the favorable hydrogen/carbon ratio of almost 4:1 as compared with gasoline when this ratio amounts to 2:1 (shift of the main combustion product from CO2 to H2O), and from the increased engine efficiency. In addition to virtually particle-free combustion, there are, in conjunction with a regulated 3-way catalyst, only very low emissions of the pollutants NOx, CO and NMHC [non-methane hydrocarbon] (these are all hydrocarbons except for methane).
The mixture regulation in an exhaust system with a jump probe upstream of the catalyst of an engine of this type is based on a threshold value control. The mixture is successively enriched until the jump probe indicates a lambda mixture lower than 1, that is to say a rich mixture, by means of a voltage which is somewhat higher than 0.5 V. A delay (holding) time duration after rich detection causes a tendential mean rich shift. A successive reduction in the control factor subsequently takes place, until the jump probe indicates a lambda mixture higher than 1, that is to say a lean mixture, by means of a voltage lower than about 0.5 V. Before the successive reduction in the control factor, the control time is shortened by means of an additional negative offset which is locked onto the mixture factor. Control subsequently takes place toward lean with an opposite sign.
For a conversion of methane in the catalyst, a slightly rich mixture in the catalyst is required. However, on account of a lengthy holding time duration after rich detection, the mean mixture can achieve only a minimal rich shift which, depending on system tolerances, is not sufficient for optimum methane conversion in the catalyst.